1. Field of the Invention
The present invention relates to a wavelength-division multiplexing optical transmitter that combines and transmits a plurality of optical signals having different wavelengths.
2. Description of Related Art
The wavelength-division multiplexing (WDM) transmission method for combining and transmitting a plurality of optical signals, which have different wavelengths, over a single optical fiber has been widely adopted in practice as a technique effective in realizing a large data-carrying capacity optical fiber.
In order to combine a plurality of wavelengths, an optical multiplexer having a plurality of input ports capable of selecting a wavelength and a sole output port is generally employed. As the optical multiplexer, for example, an arrayed waveguide grating (AWG), a combination of dielectric multilayer filters, and a combination of fiber bragg gratings (FBG) are available. As for these types of optical multiplexers, components of light other than a component having a certain wavelength are filtered even when the light components fall on the optical multiplexer, and the signal level of the light decreases by a magnitude ranging from about 25 dB to about 30 dB. When an optical transmitter delivers a specific wavelength alone, the optical transmitter and an input port of the optical multiplexer associated with the delivered wavelength are related to each other on a one-to-one correspondence on a fixed basis.
On the other hand, a wavelength tunable optical transmitter capable of tuning a wavelength to be delivered from the optical transmitter has attracted attention these days. This is because not only the number of auxiliary optical transmitters that are needed in the same number as the number of wavelengths can be drastically reduced but also wavelengths can be routed using an optical switch or filter or the settings of light paths can be flexibly varied depending on a future change or growth in the configuration of a network. Thus, a highly efficient and reliable optical network can be constructed.
A wavelength-division multiplexing transmission method that employs a wavelength tunable optical transmitter as described in “Asymmetric Reconfigurable OADMs for Next-generation Metro-DWDM Networks” (by V. Viscardi and G. Barozzi, Optical Fiber Communication Conference, the Optical Society of America, Los Angeles, Calif., 2004, TuH3) will be described in conjunction with FIG. 2. FIG. 2 shows optical add/drop multiplexing (OADM) equipment that employs a plurality of wavelength tunable optical transmitters 11. On the light extraction (drop) stage, a branching filter 10 branches out a group of wavelengths including m wavelengths. Thereafter, a demultiplexer 22 separates signals having respective wavelengths, and m optical receivers 21 receive the optical signals having the wavelengths. On the light insertion (add) stage, a wavelength-division multiplexing transmitter that comprises m wavelength tunable optical transmitters 11, a wavelength-division multiplexing unit 12 that combines m wavelengths, and a optical coupler 24 transmits a resultant optical signal.
In the above configuration or the like, a method that adopts an optical coupler (also called an optical combiner), of which input ports 12a cannot select a wavelength, as the wavelength-division multiplexing unit 12 that is connected to the wavelength tunable optical transmitters 11 is adopted in order to combine any number of wavelengths. This is because when an optical multiplexer capable of selecting a wavelength is used to multiplex signals, it means that it is impossible to communicate light, of which wavelengths are changed, with the wavelength tunable optical transmitters left connected.
Incidentally, wavelength-division multiplexing to be performed by adopting a optical coupler whose input ports are incapable of selecting a wavelength is adapted to a case where the number of wavelengths to be combined is limited. This is because when the number of wavelengths to be combined is 32, the optical coupler induces an insertion loss per wavelength that is about 15 dB. In contrast, when an AWG whose input ports are capable of selecting a wavelength is employed in the wavelength-division multiplexing unit, the insertion loss is suppressed to a range from 3 dB to 5 dB per wavelength. When the wavelength-division multiplexing unit whose input ports can select a wavelength is used in combination with wavelength tunable optical transmitters, an optical switch must be interposed between them. The optical switch switches light paths when a wavelength assigned to any of the wavelength tunable optical transmitters is changed from one to another, allows an optical signal having the changed wavelength to pass, and thus introduces the optical signal to an input port.
Japanese Unexamined Patent Application Publication No. 2004-032088 describes a fiber incorrect coupling detection system to be installed in a wavelength-division multiplexing transmitter that comprises a plurality of wavelength-fixed transmitters and an optical multiplexer which has a plurality of input ports capable of selecting a wavelength and a single output port.
Assuming that wavelength tunable optical transmitters are employed and the number of wavelengths to be combined is small, a optical coupler whose input ports are incapable of selecting a wavelength is employed in a wavelength-division multiplexing unit so that wavelengths can be changed. When the optical coupler whose input ports are incapable of selecting a wavelength is adopted, even if optical signals having the same wavelength are incorrectly routed to a plurality of input ports or an optical transmitter that has failed because of a wavelength shift is connected, erroneous light is transmitted as wavelength-multiplexed light over a transmission line and received by an opposed receiver. Consequently, since the plurality of signals having the same wavelength is received simultaneously, communication is disabled.
Moreover, when wavelength tunable optical transmitters are connected to a wavelength-division multiplexing unit capable of selecting a wavelength, an optical switch must be used in combination in order to switch light paths at the time of changing wavelengths and to introduce an optical signal having a selected wavelength to an input port that allows the optical signal having the wavelength to pass. Even in this case, if the same wavelength is incorrectly routed to a plurality of input ports, an optical transmitter that has failed because of a wavelength shift is connected, or any of the settings of the optical switch are incorrectly determined, signal light interferes with other signal light. This poses a problem in that a signal which must be received is covered with noise (coherent crosstalk).